THE EPIDERMIS 543 



comified flattened cells, which are constantly becoming 

 detached from the soft internal layer, and must needs be, 

 in some way, derived from it. But in what way ? Here 

 microscopic investigation furnishes the answer. For if 

 the soft layer is properly macerated it breaks up into 

 small masses of nucleated protoplasmic substance, that is, 

 into nucleated cells which in the innermost or deepest 

 part of the layer are columnar in form, being elongated 

 perpendicularly to the face of the dermis, on which they 

 rest, and which in the intermediate region present 

 transitions in form and other respects between the.se and 

 the shed scales. 



A thin vertical section of epidermis (see Fig. 57, p. 191) 

 in undisturbed relation with the subjacent dermis, leaves 

 not the smallest doubt (a) that the epidermis consists of 

 nothing but nucleated cells, with perhaps an infinitesimal 

 amount of cementing substance between them ; (h) that 

 from the deep to the superficial part of the dermis, the 

 cells always present a succession from columnar or sub- 

 cylindrical protoplasmic forms to flattened completely 

 comified forms. And since we know that the latter are 

 constantly being thrown off, it follows (c) that these 

 gradations of form represent cells of the deep layer 

 which are continually passing to the surface, and being 

 thrown off" there. 



What is the cause of this constant succession ? To 

 this question, also, microscopic investigation furnishes a 

 clear answer. The deeper cells are constantly growing 

 and then multiplying by a process of division in such a 

 manner that the nucleus of a cell divides into two new 

 nuclei, around each of which one half of the protoplasmic 

 body disposes itself. Thus one cell becomes two, and 

 each of these grows until it acquires its full size at the 

 expense of the nutritive matters which exude from the 

 vessels with which the dermis is abundantly supplied ; 

 such a cell in fact possesses the vital properties of ^ 

 primitive embryo cell. 



